The above mentioned play also includes the axial gap or clearance in the outer diameter area of the radial-flow final stage of a combined axial-flow/radial-flow compressor. The size of said axial gap or clearance in the outer diameter area has a considerable influence on the efficiency and the operational response of the compressor and, thus, of the entire engine. For an engine in the 1000-kw power category, e.g., the compressor efficiency will drop about 1% if said clearance is increased by 0.1 mm to 0.3 mm. The drop in efficiency in turn increases the specific fuel consumption by about 1.6% at a more than 2% drop in power per unit flow.
In conventional engine constructions featuring a single-shell compressor casing or housing and a fixed bearing arranged in the compressor intake, comparatively large changes in said tip clearance or gap will necessarily result between assembly, engine idle and full load conditions because the thermal expansion proportion of the compressor housing is larger than those of other components and hence that proportion is dominant.
Arrangement of the fixed or locating bearing of the gas generator behind the radial-flow compressor and ahead of the high-pressure or compressor drive turbine would achieve a comparatively acceptable maintenance of the gap width. However, such an arrangement requires a compromise in the maintenance and scavenging conduits for the locating bearing because the supply conduits or lines must pass through the main flow, especially in the area of the diffuser system. As a result again considerable disadvantages affecting the compressor efficiency must be tolerated because of wake turbulences caused by the relatively thick walls of the diffuser vanes required for accommodating the supply lines. Accordingly, the increased heat generation or heat exposure of a gas generator locating ball bearing necessitates the supply and scavenging of greater amounts of lubricant compared to that of a floating bearing. Therefore, the above mentioned supply lines must have sufficiently large cross-sectional flow areas which interfere with or reduce the main flow cross-sectional area to an undesirably high degree, thereby causing said comparatively pronounced wake turbulences. Hence, the arrangement of the locating bearing as described above causes considerable trouble in connection with an optimal heat dissipation from the fixed bearing when it is located in the engine environment mentioned above.
Additionally, relatively substantial difficulties are still being encountered, especially with a view to the compressor casing and/or rotor materials to be used, in the attempt to safely achieve an optimal clearance or gap in the face of the problems outlined above, especially with regard to severe load fluctuations or frequently occurring transient operating conditions.